1. Field of the Invention
The present invention relates to silica glass for use in optical systems, such as lenses and mirrors, for photolithography using the wavelength ranges of 400 nm or less and of 300 nm or less, and a method of manufacturing such a silica glass.
2. Discussion of the Related Art
In recent years, VLSI chips have been highly integrated and configured to have numerous functions. In the field of logic VLSI, a so-called "system on-chip" scheme, in which a larger system is incorporated on one chip, is becoming more and more popular. Accordingly, finer pattern manufacture and higher integration are required on the silicon wafer or like substrate of such a "system on-chip" scheme. An exposure apparatus, called a stepper, or the like has been used in the photolithography technique for exposing and transcribing a fine pattern of integrated circuits onto a wafer made of silicon or the like.
In the case of DRAMs, as technology develops from LSI to VLSI, the capacity increases from 1M.fwdarw.4M.fwdarw.16M.fwdarw.64M.fwdarw.256M.fwdarw.1G. Accordingly, the minimum line width to be produced by photolithography apparatus should be increased from 1 .mu.m.fwdarw.0.8 .mu.m.fwdarw.0.5 .mu.m 0.35 .mu.m.fwdarw.0.25 .fwdarw.m.fwdarw.0.18 .mu.m.
To cope with such a trend, a higher resolution and a deeper focal depth are required for the projection lens of the stepper. The resolution and focal depth are determined by the wavelength .lambda. of exposure light and the numerical aperture (N.A.) of the lenses.
The finer the pattern, the larger the angle of the diffraction light. Therefore, the diffraction light can not be processed unless the lens has a large N.A. Also, the shorter the wavelength .lambda. of exposing light, the smaller the angle of the diffraction light. Thus, with a shorter wavelength, a relatively smaller N.A. is acceptable.
The resolution and the focal depth are expressed by, EQU Resolution=k1.multidot..lambda./N.A.
Focal Depth=k2.multidot..lambda./(N.A.).sup.2 (where, k2 and k2 are proportional constants) According to these formulae, in order to improve the resolution, either N.A. needs to be increased, or X needs to be shortened. However, as shown in the above formula, shortening A is preferable in terms of the focal depth. Therefore, the wavelength of exposing light has been reduced from the g-line (436 nm) to the i-line (365 nm), and further to excimer laser beams of KrF (248 nm) and ArF (193 nm).
Also, the optical system installed in the stepper is constructed of a plurality of optical members such as lenses. Therefore, even if the transmittance loss at each lens is small, the cumulative effects of all the lenses may lead to decrease in light amount received at the illumination surface. Thus, very high transmittance is required for each of the optical member.
Therefore, for the wavelength band of 400 nm or less, optical glass, which is manufactured by a special method taking into account transmittance loss arising from a combination of optical members, is used. For the wavelength of 300 nm or less, synthesized silica glass or single crystal fluoride, such as CaF.sub.2, is used.
As described above, one of the properties of optical members for a photolithography technique that causes deterioration in the image contrast is a transmission loss. The transmission loss is mainly caused by light absorption and light scattering in the optical member.
The light absorption is a phenomenon caused by electron transition due to photon energy incident on the optical member. When the light absorption occurs in the optical member, the absorbed energy is converted to thermal energy. As a result, the volume of the optical member increases and the refractive index and the surface condition change accordingly. In this case, the desired resolution can not be obtained.
With regard to silica glass, in particular, the synthesized silica glass manufactured by the oxy-hydrogen flame hydrolysis method using SiCl.sub.4 as a material, there is very small amount of impurity metal. Accordingly, such a glass has superior transmittance with respect to ultraviolet light.
In general, the desired specification for the transmittance of silica glass used for the optical system of precision instruments, such as photolithography-use projection lenses and illumination lenses, is about 0.1 %/cm or less in terms of the bulk absorption.
Accordingly, deterioration in the transmittance, which may occur over a short or long period of time (referred to as "solarization"), is required to be within about 0.1 %/cm or less.
In the silica glass, especially when it is irradiated by an ArF excimer laser beam, various color centers, such as ".tbd.Si." (the E' center) and ".tbd.Si--O." (NBOHC), are generated through two-photon processes from defect precursors (.tbd.Si--Si.tbd., .tbd.Si--O--O--Si.tbd.) and SiO.sub.2 primary structure (.tbd.Si--O--Si.tbd.). Such color centers cause deterioration in the transmittance for the wavelength range in use. To deal with such two-photon absorption, increasing of molecular hydrogen concentration in the glass has been proposed in order to improve the durability against laser irradiation of the silica glass.
However, even when such a conventional silica glass, which suppresses two-photon absorption processes, is used for constructing an exposure apparatus, sufficient focusing properties and adequately high enough throughput have not been achieved.